The present invention relates to improved speed controls which provide fixed time acceleration of gas turbines and more particularly to combined cycle electric power plants in which gas turbines are operated with improved fixed time acceleration.
A gas turbine that is used for electric power generation is typically started with a diesel engine or other starting device. When the turbine reaches the ignition speed which may be 20% of synchronous speed, lightoff occurs and the starting device continues to drive the turbine until it reaches a self-sustaining speed which may be 60% of synchronous speed. Thereafter, the turbine is accelerated to synchronous speed and synchronization is performed under automatic or operator control. In combined cycle and other electric power plant applications, it is important that startup occur rapidly without unnecessary foreshortening of turbine life and that the plant availability be highly rated because of the reliability with which turbine startup can be expected to occur.
Various types of startup control have been employed for electric power gas turbines with various types of hardware technologies including relay-pneumatic, analog electropneumatic, and digital/analog electropneumatic. With respect to the different types of controls, it has been common to start a gas turbine with a temperature control loop, i.e. the turbine is caused to accelerate to synchronous speed at a rate allowed by an exhaust temperature limit and in some cases with a surge limit override. Faster startups are permitted on cold days with temperature limit startup control.
In U.S. Loft Pat. No. 3,520,133, an automatic startup control is disclosed in which analog electronic and pneumatic hardware is employed and in which a feedforward startup control is employed with temperature and acceleration limit control. In the Loft patent, the startup control is a part of the primary control system.
Further, in Ser. No. 408,962 entitled "System And Method For Starting, Synchronizing And Operating A Steam Turbine With Digital Computer Control" filed by T. Giras et al on Oct. 23, 1973 as a continuation of an earlier filed patent application and assigned to the present assignee, there is disclosed a steam turbine digital/analog control system in which a digital speed control is provided for steam valve operation. That speed control includes an integrating controller which generates a control output in response to a speed error. However, the speed control does not provide fixed time turbine acceleration and otherwise is directed to steam turbine operation.
Fixed time gas turbine acceleration is desirable generally to provide better predictability in the gas turbine and power plant operation and to make the startup time substantially independent of ambient temperature. In a digital computer/analog hybrid control set forth in a copending and coassigned patent application Ser. No. 252,131, fixed time gas turbine startup is disclosed in which a speed reference is digitally generated and applied to a speed control loop so that startup occurs in a fixed time even though day-to-day ambient temperature variations occur.
In general, the prior hybrid control provides fixed acceleration time startup with the employment of an acceleration control, i.e. a speed reference is generated with a change rate that depends on the present time and the time still available to reach synchronous speed. The present application is directed to another arrangement which achieves fixed time acceleration and which can be more desirable to users or manufacturers than the prior hybrid control in various applications.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art nor that the interpretation placed on it is unrebuttable.